It is known to produce viscoelastic fluids for use in oil well drilling and servicing, especially for the so called "drill-in" fluids used in horizontal drilling within a hydrocarbon-bearing formation. See for example the following references: "Drill-In Fluids Improve High Angle Well Production," p. 5-11, Supplemental to the Petroleum Engineer International, March, 1995; and "Soluble Bridging Particle Drilling System Generates Successful Completions In Unconsolidated Sand Reservoirs," Jay Dobson and Delton Kayga, presented at the 5th International Conference On Horizontal Well Technology, Amsterdam, The Netherlands, Jul. 14-16, 1993.
Such fluids are characterized as having a rheological profile which is shear thinning, having a high viscosity at extremely low shear rates and a low viscosity at high shear rates. Thus such fluids are pseudoplastic having a high yield stress.
This type of rheology is produced by hydrating in the fluid certain water soluble polymers. These polymers presently known are biopolymers, i.e., microbially produced polysaccharides or heteropolysaccharides, and are well known in the art.
There is a need for fluids which exhibit a high low shear rate viscosity which are shear thinning.
Chitosan is a partially or fully deacetylated form of chitin, a naturally occurring polysaccharide. Structurally, chitin is a polysaccharide consisting of beta-(1.fwdarw.4)2-acetamido-2-deoxy-D-glucose units, some of which are deacetylated. The degree of deacetylation usually varies between 8 and 15 percent, but depends on the species from which the chitin is obtained, and the method used for isolation and purification.
Chitin is not one polymer with a fixed stoichiometry, but a class of polymers of N-acetylglucosamine with different crystal structures and degrees of deacetylation, and with fairly large variability from species to species. The polysaccharide obtained by more extensive deacetylation of chitin is chitosan.
Like chitin, chitosan is a generic term for a group of polymers of acetylglucosamine, but with a degree of deactylation of between 50 and 100 percent. Chitosan is the beta-(1-4)-polysaccharide of D-glucosamine, and is structurally similar to cellulose, except that the C-2 hydroxyl group in cellulose is substituted with a primary amine group in chitosan. The large number of free amine groups (pKa=6.3) makes chitosan a polymeric weak base. Both chitin and chitosan are insoluble in water, dilute aqueous bases, and most organic solvents. However, unlike chitin, chitosan is soluble in dilute aqueous acids, usually carboxylic acids, as the chitosonium salt. Solubility in dilute aqueous acid is therefore a simple way to distinguish chitin from chitosan.
Chitosan is unique in that it is a polysaccharide containing primary amine groups. Chitosan forms water-soluble salts with many organic and inorganic acids.
It is known to prepare chitosan derivatives by attaching various groups to one or more hydroxyl groups of the chitosan, as in various cellulose derivatives, and/or in attaching various groups to the primary amino group of chitosan. Thus it is disclosed in Hall and Yalpani U.S. Pat. No. 4,424,346 that chitosan can be reacted with various aldehydes by Schiff base formation with the primary amino group. The reaction is generally conducted in the presence of a reducing agent, specifically sodium cyanoborohydride, to reduce the imino group formed to a secondary amino group, i.e., the chitosan undergoes reductive alkylation. Among the aldehyde reactants are aldose or ketose sugars, or sugars oxidizable or hydrolyzable to aldoses or ketoses, for example, glucose, galactose, arabinose, xylose, N-acetylglucosamine, lactose, cellobiose, maltose, and melibiose. These chitosan derivatives are disclosed to offer a wide range of solubility, gelling and compatibility properties.
It is also known to incorporate certain water soluble chitin derivatives into clay-containing aqueous drilling muds for controlling the viscosity, gel strength, and fluid loss of the drilling mud. See Jones U.S. Pat. No. 2,670,329.
It is well known that polysaccharides are degraded by heat as the temperature of the fluid containing them is increased. The thermal degradation of the polysaccharides decreases the viscosity of the fluid, most especially the low shear rate viscosity which provides the fluid with its desirable characteristics as many other water soluble polymers and other materials can provide the high shear rate viscosity required. Thus there is a need for enhancing the thermal stability of polysaccharide-containing aqueous fluids.